The SCP Foundation is purely fictional. It's a collection of imaginative and often creepy stories and characters that are made up for entertainment purposes and have no basis in reality.
Crossover fanfics also attract new fans. If someone is a fan of another franchise and then discovers an interesting SCP Foundation crossover, they might become interested in the SCP Foundation itself. This helps to grow the community. For instance, a 'Star Wars' and SCP crossover could draw in 'Star Wars' fans who are curious about how the two would mix.
A good way is to follow your interests. If you're interested in space exploration in science fiction, look for series like 'The Expanse'. It has books and a TV show. Start with the books to get a more in - depth understanding of the world and characters, and then watch the show to see how they've translated it. In fantasy, if you like medieval - like settings and magic, 'The Witcher' is a great option. There are books, games, and a TV show. Begin with the books, then move on to the other mediums to see how they all match together in terms of the story, characters, and the overall world - building.